Coaxial transmission line

ABSTRACT

A coaxial transmission line having an increased average power rating compared to known lines is provided. This increased power rating is achieved by improving radiant heat transfer from the inner conductor to the outer conductor.

' United States Patent Anthony Nicholas Schmitz Moorestown, NJ. 850,081

Aug. 14, 1969 Feb. 16, 1971 RCA Corporation a corporation of Delawarelnventor Appl. No. Filed Patented Assignee COAXIAL TRANSMISSION LINE 5Claims, 3 Drawing Figs.

U.S. Cl 174/ 16, 174/28,174/99,174/11O Int. Cl 1101b 7/34 Field ofSearch 174/28, 29,

16,16B,15, HS, 15C, 36, 99B, 88B, 70B, 71B, 72B,110.4,110.6,110.7

References Cited UN lTED STATES PATENTS -3/1943 Rugg et a1 174/99(B)X12/1952 Cork 174/28X 10/1967 Spindle 1. 174/28 10/1967 Shannon l74/70(B)7/1968 Whitehead 174/28 Primary Examiner- Lewis H. Myers AssistantExaminer--A. T. Grimley Att0rney-Edward J. Norton ABSTRACT: A coaxialtransmission line having an increased average power rating compared toknown lines is provided. This increased power rating is achieved byimproving radiant heat transfer from the inner conductor to the outerconductor.

TYPICAL THERMAL EMITTANCE O F-AIENTED FEB] 5197! I 2 3 4 COATINGTHICKNESS(MILS) Fig.2.

ATTENUATION 05/ 100 n. o 2

a 4 a 10 THiCKNESS OF COATING(MILS) ON BOTH CONDUCTORS Fig.3.

INVENTOR.

Anthony N. Schmitz BY ATTORNEY This invention relates to a coaxialtransmission line and more particularly to a coaxial transmission linehaving increased average power handling capability.

The maximum average power a coaxial transmission line can handle islimited by the temperature of its inner conductor. If the temperaturebecomes too high, oxidation will occur increasing the losses. The resultis further heating, causing still greater oxidation and heating andeventual thermal runaway failure of the transmission line.

It is an object of the present invention to provide an improved coaxialtransmission line having improved radiant heat transfer from the innerconductor to the outer conductor, consequently increasing the averagepower handling capability of the coaxial transmission line.

Briefly, this and other objects of the present invention are provided bya coaxial transmission line having an inner and outer conductor ofconductive material, a thin layer of low loss dielectric material on theinner surface of the outer conductor and a thin layer of low lossdielectric material on the outer surface of the inner conductor.

A detailed description follows in conjunction with the followingdrawings wherein:

FIG. 1 is a perspective view of a coaxial line in accordance with oneembodiment of applicants invention;

FIG. 2 illustrates typical thermal emissivity vs. thickness for a thinfilm of glass; and

FIG. 3 illustrates the comparison of attenuation (DB) for 100 ft. oftransmission line vs. thickness of coating of glass or similar materialon both the inner and outer conductor of the coaxial transmission line.

Heat generated in the inner conductor of a coaxial line must betransferred from the inner conductor to the outer conductor and thenceto the surrounding air or environment. The satisfactory performance ofthe line requires transfer of heat from the smaller surface area of theinner conductor to the much larger surface area of the outer conductor.The heat is transferred by radiation and convection. Convection is afunction of the geometry, temperature and properties of the surroundingmedium. Little can be done with this unless forced cooling is utilized,and this is undesirable.

The ability of a material to absorb or radiate compared to a black bodyat the same temperature and with the same area is termed the emissivityof the material. The emissivity of polished copper which isconventionally used to form the inner and outer conductor of a coaxialtransmission line is about 0.06. Most on nonconductors have anemissivity of 0.9 at normal temperatures. The placement of a thin, lowloss coating of dielectric material having an emissivity of about 0.9 onthe outside area of the inner conductor and the inside area of the outerconductor can serve to increase the heat transfer from the innerconductor to the outer conductor by radiation by 18.5 times andconsequently increase the power handling capability over 2 times theaverage power handling capability of an untreated copper line.

Referring now to FIG. 1, there is shown a coaxial line having a outerconductor 11 made, for example, of polished copper and a hollow innerconductor 13 of polished copper. Means are provided for spacing andsupporting the center conductor 13 from the outer conductor 11. In theillustrated example, this means is provided by low loss dielectric posts15, 17 and 19 placed at discrete points along the length of the coaxialline 10. Ceramic discs or anyother well knownmeans may be used for sosupporting and spacing the inner and outer conductors. The less thenumber of posts or discs the better the performance of the line due tothe attendant attenuation associated with the use of such spacing means.A thin film 25 of low loss dielectric material having an emissivitygreater than that of polished copper such as 0.9, for example, coversthe outer surface of the inner conductor 13. A second thin film 27 ofsimilar lbw loss dielectric material 27 having similar greateremissivity covers the inner surface of the outer conductor 11. Thethickness of the dielectric material 25 or 27 is made less than about 10mils. Typical thickness for low loss is in the range of mils to 2 mils.The material selected should be low conductive loss material, havinghigh emissivity and be able to withstand heat. Since the thickness ofthe material forming the films 25, 27 is made so thin, the heatconduction of the material is satisfactory for the desired applicationeven though the material itself may be a poor thermal conductor.Examples of materials usable are silicone, rubber, plastics, resin,Teflon, (polytetraflouroethylene), glass and lacquer. Selection ofthicknesses in the order of 0.5 to 2 mils provides high emissivity withthe least amount of attenuation due to the presence of the dielectricmaterial.

FIG. 2 shows a typical curve of thermal emissivity vs. thickness for asingle layer of glass on a conductor. An increase in the thicknessbeyond 2 mils results in no appreciable further increase in emissivity.By maintaining the thickness thin in the order described, the addedattenuation as illustrated in FIG. 3 is negligible, and there is betterthermal conduction through the material.

Thus, it can be seen that the film 25 on the outer surface of the innerconductor 13 by radiating heat as a function of its higher emissivity inthe direction of the outer conductor 13 enhances the transfer of heatbetween the conductors. Further, the film 27 on the inner surface of theouter conductor 10 by absorbing heat again as a function of its higheremissivity enhances the heat transfer. The presence of the films 25, 27provides additional heat transfer from the smaller surface area of theinner conductor to the much larger surface area of the outer conductor,the surface of the outer conductor being located in the ambient. In thismanner, the transmission line is able to handle more average power withthe same inner conductor temperature.

Iclaim:

1. A coaxial transmission line comprising:

an inner conductor;

an outer conductor spaced from said inner conductor in a manner such asto leave air spaces between said conductors; and

a layer of low loss dielectric material having a thickness less than l0mils covering substantially all of the outer surface of said innerconductor.

2. A coaxial transmission line comprising:

an inner conductor;

an outer conductor spaced from said inner conductor in a manner such asto leave air spaces between said conductors;

a thin cover of low loss dielectric material covering the outer surfaceof said inner conductor; and

a thin cover of low loss dielectric material covering the inner surfaceof the outer conductor.

3. The combination as claimed in claim 1 wherein said thin cover is lessthan 10 mils.

4. The combination as claimed in claim 2 wherein said thin cover is inthe range between 0.5 and 2 mils.

5. The combination as claimed in. claim 1 wherein said dielectricmaterial has an emissivity of about 0.9 at ordinary temperatures and isable to withstand heat to 250 F. without deteriorating.

1. A coaxial transmission line comprising: an inner conductor; an outerconductor spaced from said inner conductor in a manner such as to leaveair spaces between said conductors; and a layer of low loss dielectricmaterial having a thickness less than 10 mils covering substantially allof the outer surface of said inner conductor.
 2. A coaxial transmissionline comprising: an inner conductor; an outer conductor spaced from saidinner conductor in a manner such as to leave air spaces between saidconductors; a thin cover of low loss dielectric material covering theouter surface of said inner conductor; and a thin cover of low lossdielectric material covering the inner surface of the outer conductor.3. The combination as claimed in claim 1 wherein said thin cover is lessthan 10 mils.
 4. The combination as claimed in claim 2 wherein said thincover is in the range between 0.5 and 2 mils.
 5. The combination asclaimeD in claim 1 wherein said dielectric material has an emissivity ofabout 0.9 at ordinary temperatures and is able to withstand heat to 250*F. without deteriorating.